1. (Field of the Invention)
The present invention relates to an industrial manipulator, particularly a manipulator wrist structure, and a method of controlling the manipulator.
2. (Description of the Prior Art)
Industrial manipulators are currently employed in various industries. Although manipulators are many in type, most of the manipulators largely employed to accomplish factory automation are of a type resembling a human hand, an example of which is shown in FIGS. 4 to 5C for discussion of the prior art to which the present invention pertains.
Referring particularly to FIG. 4, the prior art manipulator arm shown therein comprises a first tubular shell (a first driving articulate element) 31, a second tubular shell (a second driving articulate element) 35a, an intermediate tubular shell (a driven articulate element) 35b positioned between the first tubular shell 31 and the second tubular shell 35a and a third tubular shell (a third driving articulate element) 39 positioned adjacent one of the opposite ends of the second tubular shell 35a remote from the intermediate tubular shell 35b.
The first tubular shell 31 includes a first reduction gear unit 32 incorporated therein and having a first drive axis 33, and a first drive motor 34 housed within the first tubular shell 31 and drivingly coupled with an input side of the first reduction gear unit 32. The intermediate tubular shell 35b has one end coupled with an output side of the first reduction gear unit 32 and the opposite end coupled with the second tubular shell 35a.
The second tubular shell 35a includes a second reduction gear unit 36 incorporated therein and having a second drive axis 37, and a second drive motor 38 housed within the second tubular shell 35a and adapted to drive an input side of the second reduction gear unit 36 through a drive transmission element. On the other hand, the third tubular shell 39 includes a third reduction gear unit 40 incorporated therein and having a third drive axis 41, a third drive motor 42 housed within the second tubular shell 35a and drivingly coupled with an input side of the third reduction gear unit 40 through a drive transmission element.
The third tubular shell 39 has a safety holder 14 fixedly coupled with an output side of the third reduction gear unit 40 for carrying a welding torch 13 which can revolve together with the safety holder 14 about the third drive axis 41. The welding torch 13 is connected with a cable 15 having an electric power line, a conduit and a gas hose bundled together.
In order for the manipulator arm to allow the welding torch 13 to gain access to a workpiece of a relatively complicated shape during a welding operation without interfering with a manipulator wrist, the second and third tubular shells 35a and 39 must have a size as small as possible and, particularly, the dimension L3 shown in FIGS. 5B and 5C affects the accessibility of the welding torch 13 to the workpiece. Accordingly, how small the dimension L3 is designed is one of the factors determinative of the range of application in which the welding manipulator can be effectively utilized to accomplish an intended job. For this reason, the prior art manipulator is so designed and so configured that the first and second drive axes 33 and 37 lie perpendicular to each other; the second and third drive axes 37 and 41 lie perpendicular to each other; and both of the second and third drive motors 38 and 42 are housed within the second tubular shell 35.
The operation of the prior art manipulator arm of the structure discussed above will now be described with reference to FIG. 4.
When the first drive motor 34 drives the input side of the first reduction gear unit 32 accommodated within the first tubular shell 31, the second and intermediate tubular shells 35a and 35b rotate about the first drive axis 33 in one of the opposite directions shown by .theta.1. The third tubular shell 39 can rotate about the second drive axis 37 in one of the opposite directions shown by .theta.2 when the second drive motor 38 drives the input side of the second reduction gear unit 36, accommodated within the second tubular shell 35a, through the drive transmission element. On the other hand, when the third drive motor 42 drives the input side of the third reduction gear unit 40, accommodated within the third tubular shell 39, through the drive transmission element, the safety holder 14 carrying the welding torch 13 rotates about the third drive axis 41 in one of the opposite directions shown by .theta.3.
Accordingly, it is theoretically possible for the welding torch 13 to assume any position and/or orientation relative to the workpiece by suitably selecting and/or combining directions of movement .theta.1, .theta.2 and .theta.3.
However, in the prior art manipulator arm of the type discussed above, and so long as the freedom of movement in either of the opposite directions .theta.3 is concerned, the angle through which the safety holder 14 carrying the welding torch 13 can rotate relative to the third tubular shell 39 is limited and, thus, the prior art manipulator arm is necessarily incapable of continuously assuming an arbitrarily chosen orientation. This is because, in the case of the welding manipulator such as discussed above or a paint applying manipulator, the cable 15 extending outwardly from the welding torch 13 interferes with the second tubular shell 35a when the safety holder 14 rotates in one of the opposite directions .theta.3 about the third drive axis 41.
Considering that all of the second reduction gear unit 36, the second drive motor 38, the third drive motor 42, the drive transmission element connecting the second reduction gear unit 36 and the second drive motor 38 together and the drive transmission element connecting the third reduction gear unit 40 and the third drive motor 42 together are housed within the second tubular shell 35a, the second tubular shell 35a necessarily has a square outer appearance, that is, a generally rectangular sectioned shape, as shown in FIG. 5C. Accordingly, when the cable 15 interferes with the second tubular shell 35a, the cable 15 is considerably bent with its outer sheath 15a damaged and, consequently, weld padding tends to be considerably adversely affected.
By way of example, where a welding line along which welding is desired to be effected at the workpiece lies at a location shown by B in FIG. 4, the cable 15 when moved to a position shown by 13b about the third drive axis 41 interferes with the second tubular shell 35a and, accordingly no beautiful weld padding can be obtained. Thus, the prior art welding manipulator has a problem in that the position and/or orientation of the workpiece to be welded relative to the welding manipulator have been limited.
Moreover, when it comes to effecting a fillet welding continuously along a circular welding line 19 defined between a cylindrical workpiece 17 and a base plate 18 as shown in FIG. 6 to eventually form a circular fillet joint by which the cylindrical workpiece 17 is rigidly connected to the base plate 18, the welding torch 13 has to be revolved so as to follow the circular welding line 19. In this instance, even though the workpiece is disposed in an optimum layout, interference between the cable 15 and the second tubular shell 35a necessarily takes place and, consequently, no beautiful weld padding that continues along the circular welding line 19 can be obtained.
The foregoing problems discussed in connection with the welding manipulator are equally found in the paint applying manipulator.
In view of the foregoing, it is suggested to position the cable at a location distant from the second drive axis 37 and towards the third drive axis 41. However, to position the cable at a location distant from the second drive axis 37 may result in enhancement of an angular transmission error of any one of the first and second reduction gear units 32 and 36, which in turn brings about reduction in positioning accuracy with which the tip of the welding torch 13 is positioned relative to the welding line.
To downsize or eliminate a cable guard 15a which directly interferes with the second tubular shell 35a would adversely affect the freestanding property of the cable 15 to such an extent as to result in a severe bending of a welding wire bundled in the cable 15 which would in turn bring about an adverse influence on the eventually formed weld padding.
It may be contemplated to lessen the interference by downsizing the second tubular shell 35a. However, considering that all of the second reduction gear unit 36, the second and third drive motors 38 and 42 and the drive transmission elements connecting the second and third reduction gear units 36 and 40 respectively with the second and third drive motors 38 and 43 are housed within the second tubular shell 35a, the dimensions L2 and L3 shown in FIGS. 5A to 5C can no longer be reduced economically and rationally.